[Wasm RyuJIT] Reduce Instruction Footprint for JIT Helper Calls

[adamperlin]

Currently, we duplicate the instruction sequence for loading the PEP for a helper. Ideally, we could anticipate the need for a local to hold the PEP and re-use the loaded PEP value. This may require anticipating everywhere we might emit a JIT helper call in codegen earlier on in lower, which could be a bit involved.

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[jakobbotsch]

This are also some correctness concerns listed in #126778 (comment)